Players who successfully find and free Grand Duke Ravengard during Act 3 of Baldur's Gate 3 will learn from him that an ancient dragon sleeps beneath the city, ready to protect it during a moment of dire need. To reach it, the party must light two torches in the Wyrm's Rock Prison area using Lightning damage, then pass through a hidden door.
Baldur's Gate 3: How to Solve the Wyrmway Puzzles
An ancient dragon may be Baldur's Gate's last hope, but to meet him players must first pass the trials of Balduran.